Estimation of the representative elementary volume of a fractured till: a field and groundwater modeling approach

Fractured till is often represented as an equivalent porous medium (EPM) in groundwater models. Knowledge of the representative elementary volume (REV) is necessary for proper application of an EPM model. While REV estimation and hydraulic conductivity tensor determinations are common in fractured rock studies, they are rarely applied to materials with a permeable matrix such as fractured till. This study uses field fracture measurements, model simulations, and the FracKFinder toolbox to estimate the REV and determine hydraulic conductivity tensors for the fractured, late Wisconsinan till of the Dows Formation in central Iowa (USA), at depths of 1.0–1.5, 2.0–2.5, and 3.3–3.7 m. Results indicate that the REV of the till is directly related to both fracture density and transmissivity. REV size ranges from 4 to 5 m3 at depths of 1–2 m, where fractures are densest (P32 27.5 m2/m3) and most transmissive, to 2–3 m3 at depths of 3.3–3.7 m where fractures are least dense (P32 24.4 m2/m3) and least transmissive. Fracture hydraulic conductivity is five, four, and three orders of magnitude greater than matrix hydraulic conductivity at the shallowest to deepest depths of investigation, respectively. The results also suggest that hydraulic conductivity values estimated from permeameters and large-core laboratory experiments may not be of sufficient volume to represent the REV of fractured till.